Presently, recording/reproduction of data on/from CD (Compact Disc), which constitutes the largest part of the optical disk market, is carried out using a near infrared semiconductor laser which operates in 780 nm to 820 nm wavelength band. On the other hand, recording/reproduction of data on/from DVD (Digital Versatile Disc), a rapidly-spreading, high-density optical information recording medium, requires a smaller optical spot and is therefore carried out using a red semiconductor laser which operates at a shorter wavelength in 635 nm to 680 nm band. It has been demanded for a single device to realize recording/reproduction on two kinds of discs which are based on different standards. A conventional device conceived to meet such a demand is shown in FIG. 12 (see, for example, Japanese Unexamined Patent Publication No. 11-149652). Hereinafter, the operation principle of this conventional optical pickup is described.
A semiconductor laser 1 which operates at 650 nm is used for reproduction from DVD. A light beam emitted by the semiconductor laser 1 enters a microprism 3 and is reflected by a reflection surface of the microprism 3. The reflected beam passes through a collimator lens (not shown) and an objective lens (not shown) to be collected on a disc (not shown). The light beam is reflected by the surface of the disc (not shown) and again passes through the objective lens and the collimator lens to enter a hologram element 4. The light beam is branched by the hologram element 4 and reflected by the microprism 3 to enter a plurality of photodetectors 5, 6, 7 and 8. In a reproduction operation from DVD, focus/tracking error signals and a reproduction signal are detected based on signals detected in the photodetectors 5, 6, 7 and 8.
On the other hand, a semiconductor laser 2 which operates at 780 nm is used for reproduction from CD. A light beam emitted by the semiconductor laser 2 enters a microprism 3 and is reflected by a reflection surface of the microprism 3. The reflected beam passes through the collimator lens (not shown) and the objective lens (not shown) to be collected on a disc (not shown). The light beam is reflected by the surface of the disc (not shown) and again passes through the objective lens and the collimator lens to enter a hologram element 4. The light beam is branched by the hologram element 4 and reflected by the microprism 3 to enter a plurality of photodetectors 5, 6, 7 and 8. In a reproduction operation from CD, focus/tracking error signals and a reproduction signal are detected based on signals detected in the photodetectors 5, 6, 7 and 8. Since this structure incorporates the semiconductor laser 2 which operates at 780 nm, reproduction from a write once read many CD (CD-R) is also possible.
Meanwhile, an example of a method for detecting focus/tracking error signals in an optical pickup which uses lasers of different wavelengths is disclosed in Japanese Unexamined Patent Publication No. 2001-176119.
However, in the conventional optical device shown in FIG. 12, after being reflected by the disc, the light beam of the semiconductor laser 1 and the light beam of the semiconductor laser 2 are branched by the hologram element 4 at different diffraction angles because the semiconductor laser 1 and the semiconductor laser 2 have different emission wavelengths. As a result, the diffracted light beam of the semiconductor laser 1 and the diffracted light beam of the semiconductor laser 2 form light spots at different positions over the surface of the photodetectors. Although only a component of the diffracted light, e.g., only +1st order light, can be collected at the same spot position irrespective of the difference in wavelength by particular division of the hologram element, the other diffracted light components, e.g., −1st order light, and the like, cannot be collected at the same spot positions.
Thus, in the conventional structure, the photodetector for the light beam of the semiconductor laser 1 and the photodetector for the light beam of the semiconductor laser 2 have to be provided independently of each other.
However, the independently-provided photodetectors are located at positions vicinal to each other, and therefore, it is difficult to provide another photodetector between the photodetectors. Thus, the arrangement of the photodetectors is restricted, and the location and design flexibility of the photodetectors is decreased, so that stable detection of focus/tracking error signals cannot be achieved.
Japanese Unexamined Patent Publication No. 2001-176119 discloses a focus/tracking error signal detection method which addresses the above problem. According to this method, as for each wavelength, only one direction of ±1st order diffracted light is used for detection of a focus error signal or tracking error signal. Thus, the stability of detection of the focus/tracking error signals is insufficient in view of the use efficiency of light.
The present invention was conceived in view of such circumstances. A primary objective of the present invention is to provide an optical pickup capable of detecting focus/tracking error signals such that more stable recording and reproduction are realized. Another objective of the present invention is to provide an optical pickup capable of operating on various optical information recording media that are used with semiconductor laser light of different wavelengths, characterized by the location and design flexibility of the photodetectors, and capable of detecting focus/tracking error signals such that more stable recording and reproduction are realized.